The present invention relates to devices for countering vibration in structural members, such as the frame of an aircraft; and particularly to such devices which can be dynamically tuned to adapt performance to changes in vibration frequency.
Aircraft engines can induce significant vibration into the airframe. In propeller powered planes, the propeller blades produce air pressure pulses which strike external surfaces thereby causing a time periodic vibration of the structure, at about 100 Hz for example, which vibration is transferred to other structural members of the airframe. Jet engines also produce vibration in supporting members. If left unchecked, the induced vibrations create objectionable noise in the aircraft cabin, and may result in serious fatigue of the airframe.
As a consequence vibration absorbers are attached to structural members throughout the aircraft. For example, the Fokker 50 turbo-prop airplane carries 150 frame-mounted absorbers. These devices typically are a simple spring-mass system in which a mass is attached to the airframe by a resilient member that acts as a spring that allows the mass to oscillate. Elastomeric pads and metal cantilevers have been employed as the spring. The spring-mass system is fixedly tuned to resonate at the frequency of common vibration in the structural member of the airframe to which the absorber is attached and thus optimally absorbs the vibration energy at that frequency. The absorber has a large mechanical impedance at resonance which is due mostly to a large quality factor Q. Absorption (mechanical impedance) at other frequencies diminishes as a function of the deviation from the resonant frequency.
A drawback of fixedly tuned absorbers is that the frequency of airframe vibration varies with engine speed, especially with jet powered aircraft. Although the absorber may be tuned to the vibration frequency which occurs at the nominal cruising speed of the aircraft, less that optimal vibration absorption occurs at other speeds. In addition, the tuning of elastomeric type absorbers changes with the age and temperature of the resilient material.
Therefore, it is desirable to provide an absorption system which is dynamically adaptive to variation of the vibration frequency and other factors.